U.S. Pat. No. 4,308,557 of C. B. Dieterich entitled "VIDEO DISC SYSTEM" which issued Dec. 29, 1981, describes video disc recording and playback apparatus wherein video fields on a disc are identified by digital information recorded during a selected line of the vertical interval during each field of the recorded video signal. This digital "auxiliary" information, commonly known as the system "DAXI" code, is used to control a number of video disc player operating functions during playback of the disc, such as calculation and display of elapsed playing time, lifting the pick-up stylus at the end of the program material, detection and correction of locked grooves, etc.
As proposed by Dieterich, the recorded DAXI code includes, in the order named, a Barker start code sequence, an error detection check code and a plurality of information bits which include a field number corresponding to each recorded video field. The DAXI code format desirably provides improvements in noise immunity and simplifies the decoding hardware requirements for the player.
Further improvements for decoding the DAXI data of the Dieterich format are described by Christopher in U.S. Pat. No. 4,309,721 entitled "ERROR CODING FOR VIDEO DISC SYSTEM" which issued Jan. 5, 1981. In Christopher's system the error code portion of the DAXI data is chosen so that the error code check register in the video disc player begins with the system start code in the register and, if no errors are detected after the full data message is received, also ends with the start code in the check register. This desirably simplifies the DAXI decoding logic in the video disc player.
Other U.S. Patents relating to use of DAXI code in a video disc player are No. 4,307,418 entitled "VIDEO DISC PLAYER SYSTEM FOR CORRELATING STYLUS POSITION WITH INFORMATION PREVIOUSLY DETECTED FROM DISC" which issued Jan. 26, 1982, and No. 4,313,134 entitled "TRACK ERROR CORRECTION SYSTEM AS FOR VIDEO DISC PLAYER" which issued Jan. 26, 1982, to Rustman et al.
Video disc records employing the DAXI encoding format are commericially available, for example, from RCA Corporation and CBS, Incorporated. In such records, chrominance information is recorded in the "buried subcarrier" (BSC) format proposed by D. Pritchard in U.S. Pat. No. 3,872,498. The DAXI code is recorded by pulse code modulation (PCM) of the luminance signal level during line 17 of odd fields and line 280 of even fields. The DAXI data comprises a 77 bit PCM word. The start of each bit period is synchronized with the "buried" color subcarrier frequency (about 1.53 MHz for NTSC compatible players) to facilitate subsequent detection in the player. Each DAXI word comprises a 13 bit start code (a Barker sequence for data framing) followed by a 13 bit CRC (cyclic redundancy check) error check code and ending with a 51 bit information code. Of the 51 bits in the information code, 6 provide a record band number, 18 provide a video field identification number and the remaining 27 are currently not assigned but are included to provide information capacity for future expansion or other uses of the DAXI code.
In certain video disc players for use with DAXI encoded discs, the PCM signal is obtained from the subtractive output tap of a 1-H delay comb filter in the video signal path. Each line of DAXI code is preceeded by a blank video line. As a result of the subtraction of the current and previous video lines, the resultant PCM signal is "self-referenced" and is therefore relatively unaffected by D. C. drift. This simplifies subsequent PCM detection and reduces potential bit errors which might otherwise occur without such drift compensation. Examples of video disc players wherein DAXI information is comb filtered prior to PCM detection are given in U.S. Pat. No. 4,275,416 of Dieterich entitled "PCM DETECTOR" and U.S. Pat. No. 4,278,992 of Christopher entitled "PCM DETECTOR FOR VIDEO REPRODUCER APPARATUS".
In systems of the type described, it would be desirable to provide a source of digital information for use by a device external to the player (e.g., a computer, a game accessory, etc.) to facilitate interactive uses of the video disc system. Since there are currently a total of 27 "unassigned" information bits in the DAXI code, it would appear that these bits might be used to convey "interactive" program control information to the external device. Such an approach has the virtue of simplicity (no additional PCM detectors or error check decoders are needed) but suffers from numerous technical and performance disadvantages as will now be explained.
A first problem is that the DAXI code occurs on only one line per field. This corresponds to a data transfer or "through-put" rate of only 1620 bits per second (60 fields/second times 27 "spare" bits/field). A relatively short interactive program of, say, 4000 eight-bit bytes would thus require nearly twenty seconds to read (down load) assuming no redundancy and no error correction. Taking these factors into consideration, it might take a minute or more to transfer even such a simple program to the external computer or other device.
As a second example, one cannot avoid the data transfer rate problem by simply increasing the number of DAXI code lines in a given field. This is because conventional video disc players are designed to recognize repeated field numbers as a locked groove condition and to exit the locked groove condition by effecting radial translation of the player pickup stylus (known as stylus "kicking" or "skipping"). Accordingly, to preserve compatibility with existing records and players it is required that no video field contain more than one DAXI code line.
One approach to obtaining additional data without creating problems with the player DAXI control system is described in the U.S. patent application of N. O. Ny entitled "VIDEO DISC SYSTEM HAVING TRUE AND COMPLEMENTED DIGITAL AUXILIARY INFORMATION CODES" Ser. No. 489,304 which was filed Apr. 28, 1983. In the Ny system additional data is recorded in the DAXI format but the data bits are inverted. Each data line is recorded on the video disc as a triad of lines comprising a blank line, the "inverted DAXI" line and a line of "masking data" in the order named. This format, for reasons described in detail in the Ny application is compatible with all video disc players using DAXI control systems e.g., the capacitance electronic disc (CED) players manufactured for example, by RCA Corporation, and provides up to 27 bits of additional information per triad of data lines. The Ny system desirably provides this added data capability with minimal added circuitry in the player but the overall data rate is not high and errors, while detectable with a conventional DAXI information buffer integrated circuit, are not correctable in the DAXI code format.